A device for measurement of basic sediment and water (BS&W) in a predominantly non-aqueous stream (e.g. pipeline crude oil), is disclosed in U.S. Pat. No. 4,184,952. This device is an improvement on capacitance type instruments of the art which are dependent upon the extent to which the intrinsic dielectric constant of the subject fluid varies with time. The gravity and physical composition of crude oil are two factors which determine its intrinsic dielectric constant. If one or both of these properties should vary, instruments measure the accompanying change in the dielectric constant as percent BS&W. This yields an inaccurate measurement of BS&W because instruments must be initially set to read zero BS&W as the intrinsic dielectric constant of the fluid. The capacitance type instruments of the prior art have no means for automatically correcting the zero BS&W setting to compensate for periodic variations in the oil properties mentioned. By comparison, the device of U.S. Pat. No. 4,184,952 provides for automatic compensation of BS&W measurements by producing a clean, dry sample of the line fluid for measurement of its intrinsic dielectric constant. In this way, the true BS&W content of the fluid is measured by finding the difference between the dielectric constants of the wet and dry streams.
Even though the improvement over the prior art represented by the invention of U.S. Pat. No. 4,184,952 is substantial, it now has been discovered that other improvements can be made which even further increase the efficiency and accuracy of this invention. Thus, it has been discovered that seals between the wet oil and dry oil chambers occasionally leak after extended usage. In view of the extreme sensitivity of this device, any leakage, however small, can measurably reduce the accuracy of readings. Accordingly, it is desirable to provide a means which substantially eliminates the possibility of any leakage from the wet oil chamber to the dry oil chamber.